Standard graphics X-Y plotters are known, which include one or more felt tip or ball point pens, normally of different colors. The pens may be inserted in a holder on a moving carriage arranged for relative motion with respect to a piece of paper or medium for receiving figures. It will be understood that a plotter may be designed where the paper remains fixed and the pen moves with respect to the paper, for instance in orthogonal directions nominally designated as "X" and "Y". It is also possible to keep the pen fixed and to move the paper in either one or two orthogonal directions and it is similarly possible to constrain the paper in one direction and the pen in an orthogonal direction. In the following discussion, for simplicity, reference will be made only to the orthogonal motion of the pen, however, it will be understood that the discussion also applies to the more general case of the relative motion between the pen and the medium upon which the pen writes. The carriage is controlled to move through a predetermined path to cause the pen to draw desired images on the medium.
It is also known to use a pen, such as a fountain pen, having a radially asymmetric nib in the plotter to produce calligraphic scripts. Applicants' U.S. Pat. No. 4,736,211, "Plotter Pen" issued on Apr. 5, 1988, discloses an apparatus for maintaining a radially asymmetric nib fountain pen in a predetermined desired angular orientation with respect to the paper. It is necessary to maintain angular orientation of the pen when using such a pen with an asymmetrical tip because ink flow varies with angular orientation of the pen. Thus the shape, thickness and intensity of the line being drawn will also vary.
A drawback of the fixed angular orientation device is that the line width depends solely upon the direction of motion of the pen. One effect of this constraint is shown in FIG. 1. In FIG. 1, the fixed orientation of the pen relative to the directions of pen travel X and Y is shown by the diagonal line at A, for instance inclined at an angle +45.degree. to the X axis. The path of motion of the pen is indicated by the small arrows. As the pen moves in a direction parallel to the X axis, the line width is of the relatively broad width shown at B. The line width becomes as narrow as possible when the pen moves at 45.degree. to the X axis, as at C and E. Other line widths are shown at D and F. The broadest width is possible when the pen moves at an angle of -45.degree. with respect to the X axis shown at D and F. Thus, it is not possible to vary the line width in a manner independent the direction of motion of the pen.
Similarly, it is not possible to maintain the line width constant independent the direction of the pen. With reference to the letter "O", with a fixed angle pen, necessarily two portions of the "O" at opposite ends of a diameter will be markedly narrower than corresponding portions 90.degree. around the circumference. It is impossible to write an "O" of a constant line width all the way around the letter's circumference with a pen held at a fixed angle.
It will be understood that many calligraphic hands (the calligraphers'version of fonts) require line widths that are not available at the desired location if the pen is held at a fixed angular orientation. It is possible to vary the orientation using different pens which register with the pen carrier at different angles. However, this requires frequent pen switching and does not provide as much flexibility as is required. Many forms of italic and other hands are unavailable.
Known plotters also apply the pen to the writing medium with a constant pressure. This results in a uniform line width for parallel pen motion. However, there are known calligraphic hands (in particular, those known as "copperplate") which are accomplished by the human calligrapher applying varying degrees of pressure to the pen. The varying pressure changes the imprint of the pen on the paper in a manner (discussed below), that can be finely controlled by the calligrapher. Known plotters are incapable of applying varying pressure, and thus are unable to provide copperplate hands.
Thus, the several objects of the present invention include providing:
an apparatus capable of drawing a variable line width, where the width is independent of the direction of motion of the pen;
an apparatus capable of drawing a curve of constant width where the curve is not limited to a straight line;
an apparatus capable of calligraphic writing of the copperplate style;
an apparatus capable of calligraphic writing of most hands;
an apparatus capable of both italic style and copperplate style line width variations.
an automated method of writing both italic and copperplate style line width variations.